The present invention relates to an antenna diversity arrangement comprising a plurality of antennas for providing angular diversity, and further relates to a wireless device incorporating such an antenna diversity arrangement and to a method of operating such an arrangement.
Antenna diversity is a well-known technique for mitigating the effects of multipath propagation in a radio communication system. When the signals received by two (or more) antennas are sufficiently decorrelated it has been shown that narrowband diversity gains of approximately 10 dB can be achieved.
The spatial correlation of fields in a mobile radio environment was studied in A Statistical Theory of Mobile-Radio Reception, R. J. Clarke, Bell Systems Technical Journal, Volume 47 No. 6, pages 957 to 1000. In this paper the well-known formula for the spatial envelope auto-correlation coefficient xcfx81e of a received vertically polarised wave was shown to be given by
xcfx81e=|xcfx81|2=J0(kx)xe2x80x83xe2x80x83(1)
where xcfx81 is the auto-correlation coefficient of the magnitude of the electric field, k is the wavenumber and x is the distance. This function is plotted in FIG. 1.
In communication systems that employ diversity, such as DECT (Digital Enhanced Cordless Telecommunications), it is conventional to employ two antennas to provide spatial diversity. Each antenna is designed to be omni-directional (at least for a cellular system employing substantially circular cells) and independent of the other antenna. This is achieved by separating the antennas by a large distance (which is required for good spatial decorrelation, as can be seen from FIG. 1) and, if necessary, by detuning the unused antenna. However, the large separation between the antennas places restrictions on the equipment size. Further, the potential of a dual antenna system to achieve array gain (or directivity) is not realised.
By considering the antennas as an array, an angular diversity system can be designed in which a plurality of beams are generated from a plurality of antennas. The directivity of the beams provides enhanced coverage while reducing delay spread (because of the reduced angular range over which signals are transmitted) The use of array gain enhances coverage, thereby further improving signal quality and coverage.
However, it has not hitherto been possible to design angular diversity systems having arbitrarily closely spaced antennas while maintaining low correlation between the beams. For example, PCT application WO 99/55012 presents a diversity system having two antennas separated by a third of a wavelength, with a 90xc2x0 phase shift between the feed voltages to the two antennas for the generation of directional beams. However, no account was taken of correlation coefficients in the design of this diversity system, nor of the fact that the antennas in that system did not behave as ideal point sources, with the result that the behaviour of the system will not generally be optimum.
As an alternative to angular diversity, polarisation diversity has been used with small antenna separations. However, differences in the mean power of each polarisation cause degradation of the diversity gain. Also, there is no scope for realising coverage enhancement or delay spread improvement.
An object of the present invention is to provide an antenna diversity arrangement having improved angular diversity performance from antennas which can be arbitrarily closely spaced.
According to a first aspect of the present invention there is provided a antenna diversity arrangement comprising a plurality of antennas and means for feeding each of the plurality of antennas with a signal of suitable amplitude and phase to enable the generation of a plurality of antenna beams, wherein the correlation coefficient between any pair of beams is substantially zero.
According to a second aspect of the present invention there is provided a wireless device incorporating an antenna diversity arrangement made in accordance with the present invention.
According to a third aspect of the present invention there is provided a method of operating an antenna diversity arrangement comprising a plurality of antennas, the method comprising feeding each of the plurality of antennas with a signal of suitable amplitude and phase to enable the generation of a plurality of antenna beams, wherein the correlation coefficient between any pair of beams is substantially zero.
The present invention is based on the realisation that an angular diversity arrangement having zero envelope correlation coefficient between the antennas can readily be designed using ideal point sources of radiation separated by arbitrary distances. A practical realisation, for the same antenna separations, can then be obtained by appropriate transformations to yield the required feed voltages for an array comprising dipole, monopole, helical or other antennas. The analysis methods presented here therefore enable the design of a wide range of compact antenna diversity arrangements, taking full account of mutual interactions between the individual antennas.